1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance tomography apparatus of the type wherein a basic magnetic field is produced and at least one magnetic gradient field is switched, the gradient field having a main field component that is collinear to the basic magnetic field, forming a predetermined main gradient, the gradient field also being at least one accompanying field component that is perpendicular to the main field component, and also having a linearity volume, as well as a magnetic resonance tomography apparatus for carrying out the method.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for obtaining images of the interior of a body of a living subject of examination. For this purpose, rapidly switched high-amplitude magnetic gradient fields, produced by a gradient system, are superimposed on a static basic magnetic field that is as homogenous as possible.
The gradient system includes a gradient coil system having gradient coils and a controlled gradient amplifier. One of the gradient coils produces a gradient field for a particular spatial direction; in the desirable ideal case, this gradient field has, at least inside a linearity volume, one main field component that is collinear to the basic magnetic field. The main field component forms a predeterminable main gradient that, at any arbitrary time, and independent of location, is approximately of equal magnitude, at least within the linearity volume. Since the gradient field is a chronologically variable magnetic field, the above holds for each point in time, but from one point in time to the next a strength of the main gradient is variable. The direction of the main gradient is as a rule fixedly predetermined by the design of the gradient coil.
However, based on the basic Maxwell equations, in contrast to the desirable ideal case it is not possible to construct gradient coils that produce exclusively the above-cited main field component over the linearity volume. The main field component is thereby accompanied by at least one accompanying field component that is oriented perpendicular to the main field component and that canxe2x80x94in particular given a main gradient oriented perpendicular to the basic magnetic fieldxe2x80x94have a gradient-field-type curve having an accompanying gradient that is oriented perpendicular to the main gradient. The above is described in more detail for example in the article by D. G. Norris et al., xe2x80x9cConcomitant Magnetic Field Gradients and their Effects on Imaging at Low Magnetic Field Strengths,xe2x80x9d Magnetic Resonance Imaging, vol. 8, no. 1, 1990, pages 33-37.
For the production of the gradient field, corresponding currents must be set in the gradient coil. The amplitudes of the required currents thereby amount to several 100 A. The current rise and fall rates (slew rate) are several 100 kA/s. For power supply, the gradient coil is connected to a controlled gradient amplifier.
By switching the gradient fields, stimulations can be triggered in a living examination subject during magnetic resonance image exposures. The gradient fields, which thereby act on the examination subject, are characterized by a magnetic flux density that changes over time and that produces eddy currents and induction currents in the subject of examination. The strengths of these electrical currents cited above depends, among other things, on the cross-sectional surface penetrated by the gradient field, and on the chronological change of the gradient field. The above-cited currents thereby flow through regions of the subject of examination that have different electrical conductivity, and thereby effect corresponding electrical voltages. If the voltage thereby exceeds a particular threshold, this results (undesirably) in the triggering of stimulations of the subject of examination. From German OS 42 25 592, it is for example known that given switched gradient fields the highest current or voltage values are induced at the edge or outside the linearity volume, where the field curve of the magnetic flux density of the gradient field is at a maximum, so that there the risk of stimulations is greatest at this location.
In order to avoid stimulations of this sort, it is known for example from German OS 42 25 592 and U.S. Pat. No. 5,497,773 to cover regions outside the linearity volume that are sensitive to stimulation with a closed conductor loop. In addition, it is known from U.S. Pat. No. 5,497,773 to replace the closed conductor loops with coils that can be actively supplied with current. Both the closed conductor loops and also the coils that can be actively supplied with current result in a reduction in the currents induced in the covered region. However, the coverings described above are possible only outside the linearity volume, and are not possible in edge regions of the linearity volume, because otherwise there is an adverse effect on the linearity of the gradient fields in the linearity volume and on the homogeneity of the basic magnetic field, which are important for the image quality. In addition, it is disadvantageous that, given a modification of a region to be imaged of the subject of examination, as a rule the position of the closed conductor loops and of the coils that can be actively supplied with current must also be adapted.
In addition, a method for imitating the electrical stimulations produced by gradient fields is known, for example, from German OS 199 13 547. In this document, among other things it is proposed to avoid stimulations by corresponding limitation of a chronological curve, as well as of a gradient strength, of a gradient field. However, this means that a potential efficiency of a gradient system may not be exploited at all fully.
An object of the present invention is to provide a method of the type described above for the suppression of stimulations of a subject of examination, as well as an apparatus for the execution of the method, that can be used in variable fashion and that reduces the above-cited disadvantages of the prior art.
With respect to the method, this object is inventively achieved by switching (activating) an additional magnetic field is switched that is as homogenous as possible and that extends beyond the linearity volume, and that is switched at least for a time period in which the gradient field is also switched, and that is oriented such that it reduces at least one of the field components in at least one region in which stimulation is anticipated. With respect to the apparatus for the execution of the method, the object is inventively achieved in a magnetic resonance tomography apparatus comprises an additional coil arrangement for producing the additional magnetic field, or the gradient coil system has a gradient coil for producing the gradient field, and in that the gradient coil is fashioned such that the additional magnetic field and the gradient field can be produced, or in that the apparatus for producing the additional magnetic field has an arrangement for modifying the basic magnetic field.
In this way, a field curve, responsible for stimulation, of at least one of the field components of the gradient field is reduced by the additional magnetic field in the region in which stimulation is anticipated, in order to avoid stimulations. In particular in magnetic resonance examinations of peripheral regions of a living examination subject (for example the head of a patient) there are e.g. two potential maxima of the field curve of the field component of the gradient field, the additional magnetic field reduces that maximum that causes a region of anticipated stimulation inside the subject of examination. This takes place at the expense of increasing the field curve at the second maximum, but this is located outside the subject of examination, and is therefore irrelevant to the stimulation. A region of anticipated stimulation is thereby expected in a region inside a subject of examination in which the magnitude of one of the field components has a maximum.
In an embodiment, the additional magnetic field is collinear to the field component that is to be reduced. In this way, the effect of the additional magnetic field is maximal with respect to the field component that is to be reduced.
In a further embodiment, the additional magnetic field is switched chronologically synchronously with the gradient field. In this way, stimulations are prevented during the entire duration of the examination of a living subject.
In another embodiment, upon each re-polarization (direction change), of the gradient field, the additional magnetic field is correspondingly re-polarization. This causes the additional magnetic field always to be oriented in the region of anticipated stimulation so that it counteracts the field component of the gradient field that is to be reduced, thus avoiding stimulations in the subject.
In an embodiment, at all times a magnitude or absolute value of the magnetic flux density of the additional magnetic field is set proportional to the magnitude of the strength of the main gradient. In this way, the additional magnetic field can be controlled dependent on the gradient field in a particularly simple fashion. It is thereby advantageous that the strength of an accompanying gradient is proportional to the strength of the associated main gradient, and the strength of the main gradient is easily accessible as a controllable quantity of a magnetic resonance tomography apparatus.
In a further embodiment, given a first region of anticipated stimulation in which the additional magnetic field reduces the field component, and given a second region of anticipated stimulation in which the additional magnetic field increases the field component, the magnitude of a magnetic flux density of the additional magnetic field is set such that the regions of anticipated stimulation are subjected equally to stimulation, in a manner corresponding to the respective sensitivity to stimulation of these regions.
In this way, in particular it is also possible to avoid stimulations that are caused in that the subject of examination is positioned in the apparatus in such a way that, for example, a maximum, and a minimum whose magnitude (absolute value) is equal to that of the maximum, of one of the field components of the gradient field produce two regions of anticipated stimulation that are unequally stimulated, for example as a result of the sensitivity of these regions. This is, for example, possible during an examination of an abdominal region of a patient in which the maximum or the minimum produce respective regions of anticipated stimulation in, for example, the breast region or the bone region. Since, for example, the bone region is more sensitive to stimulations than is the breast region, it makes sense to reduce the absolute field stroke in the bone region using the additional magnetic field, and at the same time to accept a larger field curve in the less sensitive breast region, so that stimulations are triggered in neither of the regions. If, in contrast, the two regions of anticipated stimulation identified above have equal sensitivity, the additional magnetic field is to be set equal to zero, and in order to prevent stimulation a strength of the gradient field and/or a switching frequency of the gradient field are possibly reduced. The above-described procedure for intentionally making the gradient field asymmetrical using an additional magnetic field is always set dependent on the position of the examination subject in the apparatus, on the sensitivities of different regions of anticipated stimulation, and on the individual sensitivity to stimulation of a subject of examination.
In another embodiment, the field component to be reduced is a gradient that is collinear to a longitudinal axis of the body of a subject of examination. In this way, in particular given an examination of peripheral regions of the subject of examination, a highly effective suppression of stimulation can be achieved using the additional magnetic field.